From 279e433b8461bd1e46af64ccfd6b187b51729c57 Mon Sep 17 00:00:00 2001
From: Kevin Stich <kevin@kstich.com>
Date: Wed, 12 Feb 2025 10:09:29 -0800
Subject: [PATCH] Add flattenAndRemoveMixins build transform

---
 docs/source-2.0/guides/smithy-build-json.rst  | 22 +++++++++++++
 .../transforms/FlattenAndRemoveMixins.java    | 26 +++++++++++++++
 .../FlattenAndRemoveMixinsTest.java           | 32 +++++++++++++++++++
 .../flatten-and-remove-mixins.smithy          | 12 +++++++
 4 files changed, 92 insertions(+)
 create mode 100644 smithy-build/src/main/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixins.java
 create mode 100644 smithy-build/src/test/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixinsTest.java
 create mode 100644 smithy-build/src/test/resources/software/amazon/smithy/build/transforms/flatten-and-remove-mixins.smithy

diff --git a/docs/source-2.0/guides/smithy-build-json.rst b/docs/source-2.0/guides/smithy-build-json.rst
index 16dbb5c92f5..543fe18b075 100644
--- a/docs/source-2.0/guides/smithy-build-json.rst
+++ b/docs/source-2.0/guides/smithy-build-json.rst
@@ -1518,6 +1518,28 @@ key is not in the provided ``keys`` list.
         }
     }
 
+.. _flattenAndRemoveMixins:
+
+flattenAndRemoveMixins
+----------------------
+
+Flattens :ref:`mixins <Mixins>` out of the model and into their local shapes.
+
+.. code-block:: json
+
+    {
+        "version": "1.0",
+        "projections": {
+            "exampleProjection": {
+                "transforms": [
+                    {
+                        "name": "flattenAndRemoveMixins"
+                    }
+                ]
+            }
+        }
+    }
+
 .. _flattenNamespaces:
 
 flattenNamespaces
diff --git a/smithy-build/src/main/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixins.java b/smithy-build/src/main/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixins.java
new file mode 100644
index 00000000000..371e13b67cb
--- /dev/null
+++ b/smithy-build/src/main/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixins.java
@@ -0,0 +1,26 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package software.amazon.smithy.build.transforms;
+
+import software.amazon.smithy.build.ProjectionTransformer;
+import software.amazon.smithy.build.TransformContext;
+import software.amazon.smithy.model.Model;
+
+/**
+ * {@code flattenAndRemoveMixins} flattens mixins out of the model.
+ */
+public final class FlattenAndRemoveMixins implements ProjectionTransformer {
+
+    @Override
+    public String getName() {
+        return "flattenAndRemoveMixins";
+    }
+
+    @Override
+    public Model transform(TransformContext context) {
+        Model model = context.getModel();
+        return context.getTransformer().flattenAndRemoveMixins(model);
+    }
+}
diff --git a/smithy-build/src/test/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixinsTest.java b/smithy-build/src/test/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixinsTest.java
new file mode 100644
index 00000000000..74ef68981b3
--- /dev/null
+++ b/smithy-build/src/test/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixinsTest.java
@@ -0,0 +1,32 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package software.amazon.smithy.build.transforms;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.contains;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+import software.amazon.smithy.model.Model;
+import software.amazon.smithy.model.shapes.ShapeId;
+import software.amazon.smithy.model.shapes.StructureShape;
+import software.amazon.smithy.model.traits.MixinTrait;
+import software.amazon.smithy.model.transform.ModelTransformer;
+
+public class FlattenAndRemoveMixinsTest {
+
+    @Test
+    void compareTransform() {
+        Model before = Model.assembler()
+                .addImport(FlattenAndRemoveMixinsTest.class.getResource("flatten-and-remove-mixins.smithy"))
+                .assemble()
+                .unwrap();
+        Model result = ModelTransformer.create().flattenAndRemoveMixins(before);
+
+        assertTrue(result.getShapesWithTrait(MixinTrait.class).isEmpty());
+        assertThat(result.expectShape(ShapeId.from("smithy.example#Foo"), StructureShape.class).getMemberNames(),
+                contains("bar", "foo"));
+    }
+}
diff --git a/smithy-build/src/test/resources/software/amazon/smithy/build/transforms/flatten-and-remove-mixins.smithy b/smithy-build/src/test/resources/software/amazon/smithy/build/transforms/flatten-and-remove-mixins.smithy
new file mode 100644
index 00000000000..eba91e7b60c
--- /dev/null
+++ b/smithy-build/src/test/resources/software/amazon/smithy/build/transforms/flatten-and-remove-mixins.smithy
@@ -0,0 +1,12 @@
+$version: "2.0"
+
+namespace smithy.example
+
+@mixin
+structure FooMixin {
+    bar: String
+}
+
+structure Foo with [FooMixin] {
+    foo: String
+}